1. Technical Field of the Invention
The present invention relates to propagation delays within transport networks of telecommunication systems, and more particularly, to a method of controlling propagation delays in a transport network by synchronizing an external node to a synchronous air interface.
2. Description of Related Art
In a cellular communication network, the synchronized air interface and an external network node, such as PLMN, PSTN, ISDN, or Packet Data Network are interconnected via a separated node such as an interworking function (IWF) or packet control unit (PCU) and a base transceiver station (BTS) over an associated transport network. However, the separation between the IWF and base transceiver station invites propagation delays over the transport network between these devices. The delays create problems with reassembling data transmitted over the transport network. For transport networks using only a single traffic channel for calls, these delays must be minimized. For transport networks using more than one traffic channel, the delays must be minimized and independent delay variations within the system must be determined in order to recreate the transmitted data stream.
One solution for combating independent time delay on different sub-channels in a fixed cellular network involves the use of a terminal adaptation function (TAF) at the mobile station, and the use at the IWF of a multi frame structure in conjunction with sub-channel memory. An inband signal is generated using redundant control bits in the CCITT V.110 frames. One bit is used for each multiframe structure and three bits are used for sub-channel numbering. This sequence resolves a delay variation of up to (n-1)/2 (where n=number of bits used in sequence) V.110 frames. However, this solution has several drawbacks.
The maximum sub-channel delay variation cannot be determined from only the algorithmic delays. Considerable delay variations may arise within the transport networks where the sub-channels may be independently routed. Furthermore, the inband signaling is transferred over the air interface where bit error rates can be very high. The error rate on the air interface and the length of the multi frame structure cause long synchronization and resynchronization times. There is also a risk of false detection. Furthermore, each of these proposed solutions have been generated with respect to the use of high-speed circuit-switched data and does not provide solutions for other types of implementations such as GPRS. Thus, alternative solutions are necessary.